Transactions IAU, Volume XXXIA Reports on Astronomy 2018-2021 © 2021 International Astronomical Union Maria Teresa Lago, ed. DOI: 00.0000/X000000000000000X COMMISSION E2 SOLAR ACTIVITY ACTIVITE SOLAIRE PRESIDENT Paul Cally VICE-PRESIDENT Manolis Georgoulis ADVISOR Lyndsay Fletcher ORGANIZING COMMITTEE Ayumi Asai, Hui Li, James McAteer, Aimee Norton TRIENNIAL REPORT 2018{2021 1. Introduction Solar activity is the business of Commission E2. It originates with the dynamo in the Sun's interior, resulting from a complex interplay of differential rotation, convection and magnetic field, and gives rise to the 22-yr sunspot cycle. Manifestations of activity range in size from coronal holes covering large fractions of the surface, through active regions with their powerful sunspots, coronal mass ejections, coronal loops, flares and nanoflares, network, etc., right down to small magnetic elements at the resolution limits of current instruments, and presumably below. As we emerge from Cycle 24, the weakest in over a century, we look back at the last three years of low solar activity but high human activity in the field. New space- and ground-based facilities are on the cusp of revolutionizing the discipline. This is a pivotal time in solar activity physics. 2. Activities of IAU Commission E2 during 2018-2021 2.1. IAUS 354 Solar and Stellar Magnetic Fields: Origins and Manifestations, Copiap´o,Chile, June 30 { July 6, 2019 This symposium, coordinated by Division E and particularly relevant to Commission E2, attracted over 200 registered participants. Its major focus was on observations and understanding of solar magnetic fields with emphasis on the context provided by other stars. Major themes included: diagnostics of solar and stellar magnetic fields; solar/stellar interior dynamics and dynamos; stellar rotation and magnetism; role of magnetic fields in solar and stellar variability; star-planet relations; formation, structure and dynamics of solar and stellar coronae and winds; mechanisms of flaring and CME activity on the Sun and stars; surface magnetic fields of cool stars; observations of solar eclipses and exoplanetary transits. For the published proceedings see Kosovichev et al. (2020) and enclosed articles. The meeting coincided with the total solar eclipse of July 2. Participants enjoyed excellent conditions for observing the eclipse from sites to the south of Copiap´o. 2.2. IAUS 365 Dynamics of Solar and Stellar Convection Zones and Atmospheres, Moscow, Russia, August 25 { 29 2020 This symposium has been postponed till August 2021 due to the COVID-19 pandemic. 1 2 DIVISION E / COMMISSION E2 2.3. IAU General Assembly XXXI, Busan, South Korea, August 16 { 27, 2021 The triennial General Assembly, including its solar activities of course, has been post- poned till August 2022 due to the COVID-19 pandemic. 3. New Facilities The advent of three major solar observational facilities, one ground-based and two in space, will transform our understanding of solar activity for many years to come. With these facilities in place, the Active Sun is hoped to give up many of its secrets. 3.1. Parker Solar Probe The Parker Solar Probe (operated by NASA/APL) launched in August 2018 and com- pleted its seventh perihelion (inside 20 R ) in January 2021. PSP will eventually dip to 6.9 R during its last five perihelia in 2024-5. First findings were announced in December 2019, with a major Astrophysical Journal supplement `Early Results from Parker Solar Probe: Ushering a New Frontier in Space Exploration' (Neugebauer 2020, and accompa- nying articles) presenting around 50 science papers resulting from the mission. The primary Science Goals are focused on the corona and solar wind and are intrinsi- cally linked to solar activity: • Trace the flow of energy that heats and accelerates the solar corona and solar wind; • Determine the structure and dynamics of the plasma and magnetic fields at the sources of the solar wind; • Explore mechanisms that accelerate and transport energetic particles. One of the first and most intriguing discoveries made by PSP was the ubiquity of `switchbacks' in the otherwise near-radial magnetic field of the solar wind (Dudok de Wit et al. 2020) that strongly affect the field dynamics and that exhibit different turbulence characteristics than their surrounds. As well as performing in situ measurements, PSP carries an imaging instrument, WISPR, designed to explore the structure of the solar corona near the Sun. 3.2. Daniel K Inouye Solar Telescope (DKIST) DKIST (operated by the US National Solar Observatory, NSO) is a four-meter solar telescope situated on the island of Maui, Hawaii, making it currently the world's largest. It saw first light in December 2019. DKIST provides unprecedented spatial, temporal and spectral resolution and dynamic range. Spatial resolutions of below 0.1 arcsec with a 5-second integration time in the near-infrared will allow researchers to resolve the fundamental scales of the magnetic solar atmosphere (Tritschler et al. 2016; Rimmele et al. 2020). The fundamental science questions addressed by DKIST include: • How are cosmic magnetic fields generated and how are they destroyed? • What role do cosmic magnetic fields play in the organization of plasma structures and the impulsive releases of energy seen ubiquitously in the universe? • What are the mechanisms responsible for (Earth-affecting) solar variability? First-generation instruments available on DKIST include: Visible Broadband Imager (VBI), Visible Spectro-Polarimeter (ViSP), Visible Tunable Filter (VTF), Diffraction- Limited Near-InfraRed Spectro-Polarimeter (DL-NIRSP), and Cryogenic Near-InfraRed Spectro- Polarimeter (Cryo-NIRSP). SOLAR ACTIVITY 3 3.3. Solar Orbiter Solar Orbiter (M¨ulleret al. 2020, operated by ESA/NASA) launched in February 2020. It will take slightly less than 2 years to reach its operational orbit. SO carries ten scientific instruments, both in situ and imaging. It will move within 60 R on a highly elliptic orbit, with an orbital inclination eventually reaching 24◦ (33◦ in the extended mission), enabling unprecedented access to the solar poles that are considered crucial to understanding the Sun's dynamo. In particular, the Polarimetric and Helioseismic Imager (PHI) onboard will produce high-cadence intensity and Doppler images suitable for helioseismic probing of the interior, especially near the poles where important clues may reside (L¨optienet al. 2014). The objective of the mission is to perform close-up, high-resolution studies of the Sun and its inner heliosphere, and answer the following questions: • How and where do the solar wind plasma and magnetic field originate in the corona? • How do solar transients drive heliospheric variability? • How do solar eruptions produce energetic particle radiation that fills the heliosphere? • How does the solar dynamo work and drive connections between the Sun and the heliosphere? An exciting early discovery of ‘campfires’ (https://www.esa.int/Science Explora tion/Space Science/Solar Orbiter/Solar Orbiter s first images reveal cam pfires on the Sun#.YGuPvmenujA.link) { multitudinous micro-flares that in concert may provide enough heating to supply the corona { was made by the Extreme Ultraviolet Imager (EUI) from Solar Orbiter's first perihelion (about 115 R ). 3.4. Other Instrumental Highlights Further instrumental and observational advances related to solar activity in the last triennium are set out in the Division E report, and will not be repeated in detail here. However, particular mention should be made of exciting solar results from a variety of radio arrays, including ALMA, LOFAR, MWA, the Jansky VLA, and the Expanded Owens Valley Solar Array. The launch of the massive two-continent Square Kilometer Array (SKA) Observatory in February 2021 also promises important advances when routine science observations begin in the late 2020s. 4. Solar Interior and Atmosphere Diagnostics Our knowledge of the solar interior continues to advance due to the availability of a wealth of continuous data acquired over multiple solar cycles and the use of sophisticated helioseismology and flow-tracing techniques. An exciting development has been the con- vincing detection of Rossby waves. This is based on work by L¨optienet al. (2018) who reported a direct detection of Rossby waves at the surface and in the outer 20 Mm of the convection zone. Both granulation-tracking and ring-diagram analysis were applied on six years of SDO/HMI data to measure horizontal flows. They found waves of radial vor- ticity propagating along the equator in the retrograde direction in the corotating frame. Dispersion relations are consistent with that of classical sectoral Rossby waves. Further confirmation was provided by Liang et al. (2019) who used time-distance helioseismology to confirm the existence of global-scale equatorial Rossby waves over the last two solar cycles, using 21 years of data from MDI and HMI, for deeper layers (down to 63 Mm). Efforts are ongoing to measure and characterize the solar meridional circulation, espe- cially the return flow with the question being whether there is a single cell or multiple cells stacked in radius, but the results are inconsistent. Gizon et al. (2020) reports a single cell in each hemisphere which contradicts previous reports of two cells (Chen and Zhao 4 DIVISION E / COMMISSION E2 2017; Zhao et al. 2013). Modeling efforts from multiple research groups are using a variety of flow amplitudes and recovery techniques to determine which meridional flow patterns can confidently be recovered. There is some indication that meridional circulation could change during or between solar cycles. Another advancement in the implementation of helioseismology techniques is the use of multiple measurement schemes, multi-skip time-distance helioseismology and holography, simultaneously to image the Sun's far side and allows far-side monitoring of active regions with higher confidence and better reliability (Zhao et al. 2019). The improved ability to monitor active regions on the far side of the Sun is useful to space weather forecasting and solar wind modeling. On an active region scale, a compact source of sunquakes was detected using acoustic holography (Lindsey et al.